Tag variants are also proposed to deal with complex structures with specialised constraints. To deal with nested NER (one mention is completely contained by the other mention), [1]} propose a joined labelling variant that each token is assigned a tag by concatenating the tags of all levels of nesting. For example, the token `HIV-1' in Figure REF  is assigned a tag `B-DNA+B-Virus', indicating that the token is the beginning token within a DNA mention and the beginning token within a Virus mention. Then the tagger is trained on the data containing the joined labels. During the inference stage, the joined labels are decoded into their original BIO format for each entity category. [2]} introduce BIOG tags for discontinuous structure without overlapping involved. The new G position indicator is used for tokens in between the components. [3]} propose to assign tags to the gaps between tokens, while still regarding the problem as a sequence labelling problem. In other words, they model the mention boundaries instead of the role of tokens in forming mentions (Figure REF ).
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